I. Field of the Invention
The present invention relates to a UMTS (Universal Mobile Telephone System) type mobile radio telecommunications system.
II. Description of the Related Art
A UMTS type system consists of a UTRAN (Universal Terrestrial Radio Access Network) type of network, mobile terminals, commonly known as user equipment (UE) devices, and a core network connected to the public switched telephone network and the Internet. A system of this kind is illustrated in FIG. 1. The UTRAN network consists of a set of radio network sub-systems RNS connected to the core network by means of an interface Iu. Each radio network sub-system RNS consists of a radio network controller RNC controlling a set of logic elements, known as Node B, by means of an interface Iub. The radio network controllers RNC are connected to each other by an interface Iur. Each Node B serves one or more cells of the network. Finally, each user equipment device UE is connected to one or more Node Bs through a radio interface Uu.
In the example of FIG. 1, the UTRAN network comprises three radio network controllers referenced RNC1, RNC2, and RNC3 and each of these radio network controllers controls two Node Bs. Four user equipment devices UE1, UE2, UE3 and UE4 are connected to these Node Bs.
The user equipment devices have two modes of operation: an idle mode and a connected mode. During the idle mode, no system resource is specifically allocated to the conveyance of data to and from the user equipment device. The user equipment device is in idle mode for example after it is powered on. In this mode, each user equipment device is identified by a core network identity which may be either the IMSI (International Mobile Subscriber Identity) of the user equipment device (which is transmitted to the core network during the first connection of the user equipment device to the UMTS system) or a TMSI (Temporary Mobile Subscriber Identity) for a circuit switch service or a PTMSI (Packet Temporary Mobile Subscriber Identity) for a packet switch service.
The user equipment device passes into connected mode when there is a request for making a connection to the system. Thus, as soon as information has to be transmitted to a user equipment device in idle mode, the core network uses the UTRAN network to send a paging message to the user equipment device concerned to ask it to pass into connected mode.
In connected mode, the user equipment device may take four states known as mobility states:                Cell_DCH for Cell Dedicated Channel;        Cell_FACH for Cell Forward Access Channel;        URA_PCH for UTRAN Registration Area Paging Channel;        Cell_PCH for Cell Paging Channel.        
These four mobility states are described in detail in the radio access network group technical specification “3GPP TS 25.331 V3.5.0”, section 7, pages 30–32 and Appendix B, pages 615–622, updated in December 2000.
In brief, in the four mobility states of the connected mode, system resources are allocated to the transmission of data in the uplink and/or in the downlink. These radio resources may or may not be shared among several user equipment devices.
In connected mode, the user equipment devices are identified by a URNTI (UTRAN Radio Network Temporary Identity) allocated by one of the Serving Radio Network Controllers (SRNC). The serving radio network controller of a user equipment device in connected mode corresponds to the radio network controller that is linked with the core network for the connection considered. For example, if we consider in FIG. 1 that the user equipment device UE3 transmits data to the core network by passing first of all through the radio network controller RNC2 and then through the radio network controller RNC1, the latter is the serving radio network controller associated with the user equipment device UE3.
In the event of the failure of one of the radio network controllers leading to a break in the connections Iu between this radio network controller and the core network and the loss of all or part of the data pertaining to the user equipment device, for example the URNTIs, all the user equipment devices having this radio network controller as the serving radio network controller are perceived by the core network as being in the idle mode. It must be noted that, when the defective radio network controller starts working properly again, it also perceives all the user equipment devices as being in idle mode. If the core network tries to set up a connection with these user equipment devices to send them data, the radio network controller will try to establish a connection using their core network identity. Since the user equipment devices are in connected mode, they will be unaware of these attempts to make connection. All these user equipment devices are therefore inaccessible until they themselves make their presence known to the network for example by an updating operation. This period of non-accessibility of the user equipment device may be very lengthy and may last several hours.
At present, the only solution to this problem lies in sending paging messages by testing all the URNTIs possible in order to connect with all the user equipment devices in connected mode having one of these identities and ask them to make their presence known to the network. This approach is not very satisfactory because it is very costly in terms of system resources (in paging channel PCH) and in time. Indeed, there may be more than 100,000 user equipment devices connected to each radio network controller.